Note: Descriptions are shown in the official language in which they were submitted.
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ALUMINUM SUBSTRATE THICK FILM HEATER
Background of the Invention
1. Field of the Invention
This invention relates to thick film resistive element heaters and more
specifically to a thick film heater with a metal substrate where the metal has
a high
coefficient of thermal expansion such as aluminum.
2. Related Art
As used herein, "Thick Film" means a metal based paste containing an
orgdnic binder and solvent, such as ESL 590 ink, nianufactured by Electro-
Science
Laboratories, Inc.,-Philadelphia, Pennsylvania ("ESL")_ "Ceramic Oxide" means
a
refractory type ceramic having a high content of oxidized metal; "MPa" means
mega
Pascals (large units of Pressure); "Coefficient of thermal expansion (10-6 /
C)"
(CTE) means micro-units of length over units of length per C or parts per
million per
C; and "W/mK" means watts per meter kelvin (units of thermal conductivity).
High
expansion metal substrates means ferrous or non-ferrous metal having a CTE of
16 x 10-6/ C or higher.
Thick film resistive element heaters are relatively thick layers of a
resistive
metal based film as compared to "thin film" technology (1-2 orders of
magnitude
thinner than thick film) and is typically applied to a glass based dielectric
insulator
layer on a metal substrate when used as a heater.
Heaters having a body or substrate made of a metal with a CTE of greater
than 16 x 10-6/ C such as high purity aluminum or high expansion stainless
steel
are desirable. This is because aluminum or other like metals have excellent
thermal
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conductivity properties which makes it an ideal substrate or body for heaters
requiring extraordinarily uniform temperature distribution. However, for
metals that
have excellent thermal conductivity and uniform heat distribution
characteristics, as
noted, it is also not unusual for these metals to have higher CTEs like
aluminum.
Conventionally, aluminum heaters are made by embedding a coil heating element
inside an aluminum cast or by putting a foil heater beneath an aluminum plate
with
an insulation material such as a mica plate in between. Aluminum heaters of
this
type can have a thinner profile than comparably rated heaters made of steel.
The
thinner profile is achievable while maintaining the desired heater performance
because of the high thermal conductivity of aluminum which is 10 - 20 times
higher
than standard 400 series stainless steel. However, as in the case of aluminum,
there is also a high CTE.
The profile of the heater can be reduced even further if the heater comprises
a metal substrate with a "thick film" heating element applied to the substrate
because
thick film technology allows precise deposition of the heating element at an
exact
location where heat is needed and intimate contact of the heating element to
the
substrate which eliminates any air gap there between. Another benefit of using
thick
film is that there is a greater flexibility of circuit designs to better
achieve uniformity in
temperature distribution and to provide precision delivery of heat for better
control
and energy savings. Also, thick film resistive elements can be made to conform
to
various contoured surfaces required for specific custom applications.
Thick film heaters are typically applied on top of a glass dielectric material
that
has already been applied on the metal substrate. It is desirable to utilize a
glass
dielectric in combination with thick film technology because glass based
materials
provide a very flat and smooth electrically insulated surface layer, glass
materials are
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not porous, and are not moisture absorbing. These characteristics of glass
materials
allow the thick film to be applied easily while achieving the desired trace
pattern and
with the correct height or elevation and width of the trace.
Thick film heating elements are desired because thick film can offer uniform
temperature distribution because of the flexibility to form various small or
intricate
heating element trace pattern designs. Therefore, a thick film on an aluminum
substrate would be very useful if it could be made to work because of
aluminum's
thermal performance characteristics. So far the prior art teaches the use of a
glass
based dielectric when using thick film over a metal substrate, but that will
not work
when using aluminum as the substrate metal or other metals having a high CTE
relative to the typical glass dielectric utilized with thick film. Therefore,
even though
the therrr7al performance nf aluminum is riPsirahle, the high CTE is not
compatible
with a glass based dielectric. As seen in industry, thick film heaters on
metal
substrates use glass dielectric material to serve as an insulation between the
thick
film and the metal substrate, usually 400 series stainless steel which has a
CTE of
12 x 10-6/ C. The reason why aluminum or other higher CTE metals are
problematic is aluminum has a much higher thermal expansion coefficient than
glass
used for 400 series stainless steel and therefore causes cracking in the glass
dielectric material when heating or cooling occurs. The cracking causes opens
in the
resistive heating film resulting in a defective heater. Cracking typically
occurs when
the aluminum substrate is cooling down and contracting after the temperature
has
been raised. A second problem is that the typical printing method for applying
such
a dielectric is screen printing which requires a firing post-process for the
curing of the
dielectric. The melting point of aluminum is about 600 C. Therefore, if a
glass
dielectric is utilized, it must have a lower melting point than 600 C in order
to be
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properly fired for adequate curing. A glass having a low melting point of 600
C can
be found, but the final heater design will be limited to a low operating
temperature
(below 400 C). This is because the softening temperature of a glass dielectric
is
usually 200 C or more lower than the melting temperature (hypothetically 600 C
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order to work with aluminum). Also, when glass reaches its transition
temperature,
which is 50-100 C below the softening temperature, the glass will
significantly loose
its insulation resistance properties. Therefore, just above the softening
temperature,
the glass will significantly loose its insulation resistance properties, so
the heater is
limited to temperatures below 300 C. This renders an aluminum-glass heater
design
useless for many applications. In addition, the dielectric cracking problem is
not
resolved by choosing a glass dielectric with a lower melting point. A third
problem is
that if a glass with a lower melting point is chosen then the firing
temperature to cure
the thick film element applied on top of the dielectric is limited to that of
the glass.
Therefore a special thick film must be found that has a lower curing or
sintering
temperature.
The above problems have prevented the use of thick film heater elements on
aluminum substrates because, even if a thick film with a lower melting point
(lower
than the melting point of the glass dielectric chosen) is found and utilized,
the
resulting operating temperature of the heater would be useless for many
operating
temperatures and the dielectric cracking problem is still not resolved because
the
difference in the coefficient of thermal expansion still exists. Also, a glass
based
dielectric with such a low melting point will have poor insulation performance
at the
higher operating temperatures and insulation breakdown is likely.
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Conventional wisdom then is that aluminum or other higher CTE metals like
high expansion stainless steel is simply an incompatible substrate for thick
film
heaters.
Summary of the Invention
It is in view of the above problems that the present invention was developed.
The invention thus has as an aspect to provide a thick film resistive heating
element
disposed on an aluminum substrate or substrate of a higher CTE metal relative
to
the CTE of the typical glass based dielectric utilized with thick film by
interposing an
alumina dielectric, or other comparable ceramic oxide, insulator there
between.
It is another aspect to pr,ovide more efficient heating in a thick film
heater.
It is also an aspect to provide better temperature conlrol capability far
ttrick filtn
hcaters.
It is yet another aspect to provide a faster responding thick film heater.
ft is a further aspect to provide a more uniform surface temperature
distribution
for thick film heaters.
It is a still further aspect to eliminate the glass dielectric so as to not be
limited
by the low melting or processing temperature of the glass dielectric.
Some embodiments of the invention have solved the puzzle posed by the prior
art and satisfy all the
above objects by providing a method and apparatus for a thick film heater
utilizing an
aluminum substrate or a substrate made of metals having a CTE of greater than
16 x
10-6/ C which were previously known to be incompatible with thick film
technology.
The inventors have gone against conventional wisdom and by doing so have found
a
resolution to the problems outiined above. The inventors have developed an
aluminum substrate heater with a refractory ceramic oxide dielectric, such as
alumina, applied with a thermal bonding process such as a plasma spray process
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whereby firing is not required to cure or densify the dielectric and a thick
film resistive
trace heating element applied on the dielectric. The elimination of firing is
a major
advance allowing much more flexibility in design of the thick film. In
addition, even
when the thick film resistive trace is fired, the alumina or other ceramic
oxide
material can withstand the temperature shock and the expansions and
contractions
of aluminum. The same holds true when the heater is in normal operation. This
heater is expected to be a key breakthrough in thick film heater design.
The inventor has also discovered that if the glass based insulative over glaze
top layer that is typically applied over thick film resistive element heaters,
is replaced
by a ceramic oxide over coat insulative top layer, the heater performance at
the
upper temperature range is improved. The improved performance is due to better
high temperature performance characteristics of ceramic oxides such as high
melting
point, insulation resistance, rigidity and fracture strength.
The inventor has theoretically and empirically determined that alumina and
other ceramic oxides with similar properties can withstand the temperature
shock
when the thick film is fired and can withstand the contractions and expansions
of an
aluminum substrate or other higher CTE metals during normal usage.
It should be noted that choosing a metal that has superior thermal
performance parameters is only one of many reasons why a metal is chosen for a
heater design. A metal may also be chosen because of its compatibility with
the
environment in which it is to operate or because of some other charateristic
that
makes it the preferred metal. However, the preferred metal may also happen to
have a higher CTE relative to the typical glass based dielectric utilized with
thick film
technology. Therefore, the heater designer may have to rule out the preferred
metal
because the designer also desires to utilize a thick film heater element
because of
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the desired profile of the heater and/or because of the
surface on which the heater element must be applied. The
designer in such circumstances is forced to make a design
decision as to which is most important, utilization of thick
film or the preferred metal.
This is then a key breakthrough that will open the
door to numerous subsequent advances in thick film heater
design and because of that will lead to many advances in the
design of small heater parts in many future devices.
It was discovered, as part of an aspect of the
invention, that greater temperature control and thermal
efficiency can be achieved with the use of an aluminum
substrate as compared to stainless steel.
it was also discovered that a glass based
dielectric for a thick film heater on a metal substrate is
not the only option.
According to one aspect of the present invention,
there is provided a resistive element heater comprising: a
metal substrate having a CTE greater than 16 x 10-6/ C; a
ceramic oxide dielectric layer bonded on said substrate; and
a thick film resistive element layer bonded over said
dielectric, with the dielectric layer separating the
substrate and element layer.
According to another aspect of the present
invention, there is provided the method of making a
resistive element heater comprising the steps of: forming a
metal substrate from metal stock having a CTE greater than
16 x 10-6/ C; roughening t:he surface of the metal substrate;
applying ceramic oxide dielectric powders by thermally
bonding onto the roughened surface forming a densified
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dielectric layer; printing a thick film resistive layer on
said dielectric; and applying ceramic oxide overcoat by
thermally bonding onto the said resistive layer and said
dielectric layer.
According to still another aspect of the present
invention, there is provided a resistive element heater
comprising: a substrate of metal with a CTE greater than
16 x 10-6/ C having a surface created by roughening a surface
of a piece of metal stock having a CTE greater than
16 x 10-6/ C; a dielectric layer of ceramic oxide deposited
on the roughened substrate by thermal bonding; and a
resistive layer deposited on the dielectric layer by
printing a noble metal paste containing an organic binder
and solvent over said dielectric layer.
Brief Description of the Drawing
The advantages of this invention will be better
understood by referring to the accompanying drawing, in
which
Fig. 1 shows a vertical cross section of the
layers of the aluminum substrate heating device.
Fig. 2 shows ari alternative heater embodiment.
Fig. 3 shows an alternative heater embodiment.
Description of the Invention
Referring first to Fig. 1, a vertical cross
section of the high CTE nletal substrate like aluminum
heating device 100 is shown. A high CTE metal (such as
aluminum) plate 102 having a flat surface 104 that has been
roughened by a method of sandblasting or particle blasting
or other appropriate method and that forms the substrate for
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the heating device. The plate in its preferred embodiment
is
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high purity aluminum but depending on the application an aluminum alloy may be
utilized containing elements such as Mg, Si, Cu, or other elements of like
properties.
Also, other metals having high CTEs above 16 x 10-6/ C may be chosen.
The substrate can have a surface roughness in the range from
about 100 }.zin. to about 200 uin. The roughened surface
makes for better adherence of the dielectric material
because of the increased surface area.
A thermally applied (such as plasma sprayed) dielectric layer 106 of ceramic
oxide (a ceramic containing an oxidized metal) is applied over the roughened
substrate surface. Alumina (AI203) is an example of a ceramic oxide that can
be
utilized and is considered the preferred embodiment. The alumina prior to
introduction into the plasma spray or other thermal application is in the form
of A1203
powders which is preferred to have a purity greater than 99% and a particle
size
within the range between from about 0.1 to 10mm and having a mean size
wittiiri tiie
range between from about 1 to 3 mm, but these parameters may vary dependent on
the application. The thickness of the dielectric coating applied is preferred
to be
within the range between from about 75 to 250 mm, but can vary dependent on
the
application. However, zirconia (ZrO7) is also a ceramic oxide that can be
utilized or
other ceramic oxides of similar characteristics.
Traditionally the dielectric layer was made of glass or glass ceramics by
screen printing followed by a firing process to burn off the organic binder
and
consolidate and densify the glass dielectric to minimize the porosity. The
purpose of
-minimizing the porosity was to reduce the possibility of insulation breakdown
at high
temperatures or high voltages. Also, excess porosity may allow the thick film
to
penetrate through the dielectric layer thereby shorting to the metal
substrate.
However, as noted in the Related Art section above, the traditional glass or
glass
based dielectric is not compatible when using a thick film heating element
over an
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aluminum substrate due to the incompatibility of the coefficients of thermal
expansion of the aluminum, glass and thick film during burn off or actual
operation.
The glass or glass based dielectric is prone to crack under such conditions.
The key
characteristics of the dielectric for adequate performance when applied over
aluminum are fracture toughness, coefficient of thermal expansion and melting
point.
Ceramic oxides that fall within the following range is preferred:
for CTE: 6 x 10-6/ C to 19 x 10-6/ C
for fracture strength: greater than 100 MPa
for melting point: greater than 600 C
However, these parameters may vary dep ndant on what aluminum alloy or other
high CTE metal that is chosen.
A silk screened metal based paste containing glass, an organic binder and
solvent, such as, for example, ESL 590 ink available commercially from the
manufacturer ESL, (thick film) heating element circuit pattem 108 is applied
over the
dielectric layer 106. -The heating element is preferably made of pure Ag or an
Ag/Pd
alloy with elements such as glass with a melting temperature of below 600 C.
The
thick film is dried at a high temperature, approximately 150 C, for
approximately 40
minutes to remove the solvent and the thick film is subsequently fired for
approximately 10 to 15 minutes at a high temperature approximately 580 C in
order
to consolidate the thick film and to provide for adequate bonding to the
alumina
dielectric. The thick film thickness once applied can be in the range from
about
between the range 5 to 30mm and a resistivity in the range of about between
3mW
to 1000W per square inch. The thick film can be printed over the dielectric by
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various methods to achieve the desired result such as thermal spraying, laser
cading, or direct writing
The heating element circuit pattern terminates at terminal foils 110 by
bonding
the circuit pattern terminals to terminal foils 110 with a bonding agent such
as a
brazing alloy or a fritted conductive noble metal paste which overlay the
termination
lead ends of the circuit pattern. The thick film circuit pattern is attached
by a brazing
alloy bonding agent as a preferred embodiment. An insulative over coat top
layer
114 is then applied over the heater element circuit pattern. A preferred over
coat
material is a ceramic oxide such as alumina (A1243) or zirconia (Z02) or
another
ceramic oxide with comparable thermal and insulation properties. The ceramic
oxide
over coat is applied by using a plasma spray coating process or other standard
applic..~tinn prncAss. The thermal and strength properties of the ceramic
oxide over
coat is preferably the same as the properties of the ceramic oxide used for
the
dielectric layer. However, the thickness and surface texture of the dielectric
layer
and that of the over coat layer may differ.
If an over glaze top iayer is chosen, it should be noted that for thick film
heaters the insulative top layer 114 is typically glass based. It is typically
a silk
screened over glaze paste top layer 114 containing glass, an organic binder
and
solvent (such as, for example, ESL 4771G ink made by ESL) that is applied
(thick
film over-glaze) over the heater element circuit pattem. The over-glaze is
glass
based and preferably contains major components such as Si, B, 0, Al, Pb,
alkaline
earth elements (Mg, Ca, Sr, Ba) and alkaline elements (Li, Na, K).
However, if a glass based over glaze is used as an insulative top layer 114,
the maximum operating temperature may be limited. As noted above, using a
glass
based dielectric layer to serve as an insulation between a thick film heating
element
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circuit pattern and an aluminum substrate is problematic. This is because
aluminum
has a very high coefficient of thermal expansion (CTE), much higher than that
of
glass. The mismatch in CTE between the glass dielectric layer and a metal
substrate having a high CTE causes cracking in the dielectric layer during
firing and
actual operation.
An analysis of the design, however, suggests that the use of a glass over
glaze as an insulative top layer is not as critical as use of a glass
dielectric over an
aluminum substrate. This is because the glass based top layer is not applied
directly
to the aluminum substrate. Thus, the change in CTE between the top layer and
the
adjacent layers (thick film resistive element layer and ceramic oxide
dielectric layer)
is not as large as that between a glass dielectric and an aluminum substrate.
Also,
insulation resistance is not as critical as the dielectric layer on the
substrate from a
leakage point of view. Therefore the expansion shock caused by the aluminum
substrate is not transduced directly to the top layer.
In summary, the glass over glaze top layer is applied by a silkscreen process
and thus must be fired in order to cure. Thus the firing temperature and the
possible
high operating temperatures of a heater and the resulting cool down may induce
cracking even in the top layer because of the high CTE of an aluminum
substrate.
Therefore, even though cracking is less likely when a glass based material is
used
as a top layer as oppose to when it is used as a dielectric layer, a ceramic
oxide
material as an insulative top layer remains the preferred embodiment.
Referencing Figs. 2 and 3, other heater body and heater element circuit
pattern embodiments are shown. In Fig. 2 a circuit pattern is shown applied
over a
flat substrate. In Fig. 3 a circuit pattern is shown over a tubular substrate.
A plurality
of other substrate and circuit pattern designs may be implemented. For
example,
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the substrate could have irregular contours and/or the circuit patterns could
have
irregular continuous traces.
In view of the foregoing, it will be seen that the stated objects of the
invention
are achieved. The above description explains the principles of the invention
and its
practical application to thereby enable others skilled in the art to best
utilize the
invention in various embodiments and with various modifications as are suited
to the
particular use contemplated. As various modifications could be made in the
constructions and methods herein described and illustrated without departing
from
the scope of the invention, it is intended that all matter contained in the
foregoing
description shall be interpreted as illustrative rather than limiting. Thus,
the breadth
and scope of the present invention should not be limited by any of the above
described exemplary embodiments, but should be defined only in accordance with
the following claims appended hereto and their equivalents.
All patents, if any, referenced herein are incorporated in their entirety for
purposes of background information and additional enablement.
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